Initial photos have been released of the building site where HS2 tunnels will dive under the M25 and tunnel their way beneath the Chilterns towards Birmingham.
The South Portal Chalfont Lane site will be the biggest construction site on the project. From here, one team will launch the tunnels northwards, while another heads towards London on the adjacent Colne Valley Viaduct.
Around 350 staff from HS2’s main works contractor, Align JV are on site now, and at its peak, around 1200 people are expected to be employed in the design and construction of the Chiltern tunnels and the viaduct.
The new images show the excavation of the launch site and the start of work to build the concrete precast plant where more than 118,000 tunnel segments will be made.
A concrete wall is needed to stabilise the excavated site and give the tunnel boring machines a ring to hook into when they start digging. The TBMs need to start this deep down as they need a certain amount of soil above them to be able to start tunnelling.
Two tunnel boring machines – currently being manufactured in Germany – will dig the 10-mile-long Chiltern tunnels and will launch from this site just inside the M25 motoroway, not far from Gerards Cross. The ‘twin bore’ Chiltern tunnels will be the longest and deepest tunnel bores on route, with separate northbound and southbound tunnels and five ventilation shafts.
Material excavated from the tunnels will be reused as landscaping on site.
Just to the south of the tunnel portal, the Colne Valley Viaduct – designed by Align, working with its design partners Jacobs and Ingerop-Rendel, and the architect Grimshaw – will be the longest railway viaduct in the UK. At 3.4km long, it will carry HS2 across the Grand Union Canal and surrounding lakes.
Further south, the line will go into tunnel again before arriving at Old Oak Common and London Euston.